Solder Mask in Board Assembly
Solder mask is a chemical layer on an unpopulated circuit board with openings for the areas solder will cover and covering the rest. It protects against accidental solder bridging during assembly and helps the PCB remain functional over its life. The significance of the coating is widely recognized by those who fabricate or troubleshoot boards and it plays a vital role in board assembly.
While many think of it as green, the mask can be a variety of colors including white, black, red and more. Different colors are used for specialized applications like implied ground planes or forked circuits, or simply to make it easier to identify multiple boards during inspection and rework. Proper coloring is critical for quality and manufacturability.
The mask also serves to insulate the copper traces and pads, improve component placement accuracy, prevent shorts between closely spaced traces or pads and enhance electrical reliability. It also protects the copper from contamination and oxidation. Oxidation is a problem because it causes copper to lose its electrical properties, leading to malfunction. The mask prevents oxidation by sealing the copper away from moisture and oxygen. This is especially important for electronic devices that are exposed to the elements, like a computer or cell phone. It also prevents whiskers, thin filaments that appear on the surface of copper and are a common cause of short circuits.
The Significance of Solder Mask in Board Assembly
In addition to preventing oxidation and shorts, the mask protects the board from environmental contaminants and corrosion, and extends its usable lifetime. This is especially important for high-reliability, mission-critical designs that must be tested and approved under stringent environmental conditions.
During the manufacturing process, the solder mask is applied to the production panel in either liquid or dry form. The panels are then cured with a high intensity UV light that passes through a clear film that contains darkened apertures printed on it for areas where the mask is not wanted. This prevents the light from curing the mask in these areas and allows it to be washed off during the developing process.
Liquid solder mask is sprayed on the PCB surface in a curtain pattern, ensuring even coverage and thickness. It can be printed in a variety of colors, and is often matched to the color of the copper on the board for consistency. Another popular method is inkjet printing which uses precision print heads to deposit the mask material layer by layer, allowing for more flexibility and customization.
The panels are then UV-exposed with the correct mask pattern and developed. Once the mask is cured, it is ready for assembly. A good rule of thumb is to place a solder mask dam between each surface mount pad, as this will help prevent solder from accidentally bridging neighboring pads during assembly and reduce the amount of rework required to remove shorts. It’s also recommended that the fabrication drawing note any areas where the mask should be oversized to discourage solder mask encroachment on the pads.
